1. Field of the Invention
This invention is related generally to AC power distribution systems, and in particular to AC power controller systems that control the application of AC operating power to induction motors.
2. Description of the Related Art
Spurious noise signals, including harmonic currents, background noise and spike impulse noise are developed on AC power distribution lines. Such noise signals can originate from the power source, the distribution network, local and remote loads coupled to the network, lightning strikes and distribution equipment malfunction. The AC supply current delivered from a public utility is not a pure sine wave and contains harmonics that interfere with proper operation of connected equipment. Additionally, noise and switching transients may be introduced from active loads. By way of example, if a branch is loaded by an electronic dimmer and lamp, the dimmer will “chop” the 60 Hz AC power waveform at a high frequency to reduce the lighting intensity. This will introduce harmonics and high frequency noise on the power distribution conductors.
Such noise is not constant with respect to time, and it also varies from place to place in the power distribution network. Moreover, a typical AC power line network distributes power to a variety of electrical load devices. Each load can conduct a significant level of noise and harmonic currents back onto the power line, causing distortion of the power waveform. Different loads and control devices produce different types and degrees of distortion that may interfere with the operation of the equipment and machines that are being supplied by the distribution network.
The amount of electric power used by machinery and the machinery itself can be affected by waveform distortions present in a power distribution system. Elimination or control of the distortions may provide a substantial cost savings with respect to electrical energy consumption, and a cost savings with respect to machinery failure and repair or replacement. Thus, mitigation and reduction of harmonic distortions in AC power distribution systems can result in a substantial energy cost savings for industrial customers.
In the context of AC power distribution systems, linear electrical loads are load devices which, in steady state operation, present essentially constant impedance to the power source throughout the cycle of the applied voltage. An example of a linear load is an AC induction motor that applies torque to a constant (time invariant) mechanical load. Non-linear loads are loads that draw current discontinuously or whose impedance varies throughout the cycle of the input AC sine wave. Examples of nonlinear loads in an industrial distribution system include arc lighting, welding machines, variable frequency drive converter power supplies, switched-mode power supplies and induction motors that are applying torque to time-varying mechanical loads.
Harmonic currents produced by non-linear loads in an electrical distribution system flow away from the non-linear source and toward the distribution system power supply. The injection of harmonic currents into the power distribution system can cause overheating of transformers and high neutral currents in three phase, grounded four wire systems. As harmonic currents flow through the distribution system, voltage drops are produced for each individual harmonic, causing distortion of the applied voltage waveform, which is applied to all loads connected to the distribution bus.
Harmonic distortion of the voltage waveform affects AC induction motor performance by inducing harmonic fluxes in the motor magnetic circuit. These harmonic fluxes cause heat build-up and additional losses in the motor magnetic core, which reduce power transfer efficiency. Inductive heating effects increase generally in proportion to the square of the harmonic current. Induction motors can be damaged or degraded by harmonic current heating if the supply voltage is distorted. Negative sequence harmonic currents operate to reduce motor torque output. The combination of these effects reduce power transfer efficiency and can cause motors to overheat and burn out.
Harmonic fluxes in the motor windings are either positive, negative or zero sequence depending on the number or order of the harmonic distortion that created them. Positive sequence harmonic magnetic fields (flux) will rotate in the direction of the synchronous field. Negative sequence harmonic flux will rotate in opposition to the synchronous field, thereby reducing torque and increasing overall current demand. Zero sequence harmonic flux will not produce a rotating field, but still will induce additional heat in the stator windings as it flows through the motor magnetic circuit.
Industrial power distribution systems supply AC operating power to connected machinery and devices that produce some harmonic distortion of the AC voltage waveform. Each harmonic of the fundamental frequency, depending on whether it is a positive, negative, or zero sequence, and its percentage of the fundamental, can have an adverse affect on motor performance and temperature rise, as well as increase the energy costs of electrical service that is charged by the utility service provider. Electric utilities must generate service capacity adequate to meet the expected peak demand, kVA (kilovolt amps apparent power), whether or not the customer is using that current efficiently. The ratio of kW (real active Power) to kVA (apparent power) is called the load power factor. Most utilities charge a penalty to customers when the customer's total load power factor is low.
Apparent power can be larger than real power when non-linear loads are present. Non-linear loads produce harmonic currents that circulate back through the branch distribution transformer and into the distribution network. Harmonic current adds to the RMS value of the fundamental current supplied to the load, but does not provide any useful power. Using the definition for total power factor, the real kW is essentially that of the fundamental (60 Hz) AC waveform only, while the RMS value of the apparent kVA is greater because of the presence of the harmonic current components.
A low kW/kVA power factor rating can be the result of either a significant phase difference between the voltage and current at the motor load terminals, or it can be due to a high harmonic content or a distorted/discontinuous current waveform. An unacceptable load current phase angle difference can be expected because of the high inductive impedance presented by the stator windings of an induction motor. A distorted current waveform will also be the result of an induction motor that is applying torque to a non-linear load. When the induction motor is operating under discontinuous load conditions, or when the load is non-linear, high harmonic currents will result, degrading motor performance and reducing power factor.
Some power factor correction can be achieved by the addition of capacitors connected across the induction motor stator windings. The resulting capacitive current is leading current that cancels the lagging inductive current flowing from the supply, thus improving the power factor when the induction motor is driving a linear load. For example, KVAR (Kilovolt Ampere Reactive) capacitors may be installed to correct low power factor caused by the high inductance of stator windings. Harmonic currents produced in the load circuit or that are conducted along the branch power distribution line from remote non-linear sources may find a resonance with the KVAR capacitors, and the resulting high current may cause the capacitors to fail. These harmonic currents, when combined with the inductive reactance of the distribution network, can also cause premature motor failure due to excessive current flow, heat build-up and random breaker tripping.
Controllers for reducing energy consumption of AC induction motors have been developed or proposed. One class of such devices uses a measure of the power factor of the AC induction motor to generate a feedback signal that is used for controlling the amount of power delivered to the motor. The control signal is adjusted from time-to-time to reduce the average power applied to the motor during light loading in order to maintain sufficient rotor slip for operation with a relatively high power factor and good power transfer efficiency.
Various problems arise in the operation of conventional controllers, particularly when controlling power applied to non-linear loads. For example, complex power control factors are presented by the operation of AC induction motors that drive pumping units (pump jacks) used to lift fluids from underground formations. Such pumping units are alternately loaded by a pumping rod, the weight load of the formation fluid column, and opposing counter-weights twice each pumping cycle. Moreover, twice each cycle the opposing loads balance and the motor is thus unloaded twice each cycle. The constantly changing load between peak minimum and maximum values creates severe control difficulties for power factor control systems which must continuously adjust the power delivery to maintain optimum motor efficiency and economy.
Currently, thyristor switches are in use in conventional controllers for controlling the AC power supplied to induction motor loads, for example in the AC power controller disclosed in U.S. Pat. No. 6,400,119. Because of the fast on-off switching action (fast dv/dt) of the thyristors, high peak voltage and high switching frequency, the input current on the supply side of the power controller becomes distorted with high frequency switching transients, which cause an increase of harmonic components in the AC power delivered to the induction motor. Moreover, spurious noise and harmonic currents from remote sources that are conducted down the branch distribution circuit can interfere with the proper switching operation of the controller itself, resulting in loss of power control.
These factors not only reduce the power factor of the branch load, but also interfere with motor operation and inject harmonic currents back through the power distribution branch and into the distribution network. Moreover, controller-generated harmonic distortion increases the RMS value of the load current in the power distribution branch, on which the utility service fees are based, thus increasing the customer's energy costs.